1. Field of the Invention
The present invention relates to an electrophotographic toner which is used in apparatuses utilizing an electrophotographic process such as a copying machine, printer, facsimile and the like, especially in a color copying machine (hereinafter, this toner is sometimes simply abbreviated to "toner"), a process for producing an electrophotographic toner, an electrophotographic developer (hereinafter, this developer is sometimes simply abbreviated to "developer") and a method for forming an image.
2. Description of the Related Art
In an electrophotographic process, a fixed image is formed through a plurality of processes in which a latent image is electrically formed by various means on a photosensitive material utilizing a photoconductive substance, this latent image is developed using a toner, the toner latent image on the photosensitive material is transferred onto a transfer material such as paper and the like to manifest a toner image via or not via an intermediate transfer material, then, this transferred image is fixed on the transfer material such as paper and the like. Recently, owing to the development of apparatuses and the spread of communication networks in today's information-oriented society, electrophotographic processes are used not only in copying machines but also in printers widely, and there are increasingly strict requirements for compactness, lightness and high speed and high reliability of the apparatuses utilized.
Particularly, in the case of color electrophotography, it is required that an image formed has high quality and high color developing ability. For obtaining an image having high quality and high color developing ability, the toner has to be melted sufficiently and the surface of an image after fixation should be smooth in light transmittance, gloss and the like. For this reason, the fixation step in an electrophotographic process becomes very important.
As a contact type fixation method normally used as a fixation method, a method utilizing heat and pressure in fixing (hereinafter, referred to as "heat pressuring method") is general. In this heat and pressure method, since the surface of a fixing member and a toner image on a transfer material are in contact under pressure, heat efficiency is extremely excellent and quick fixation can be conducted, and in particular, this method is very effective in high speed electrophotographic copying machines.
However, in the above-described heat pressuring method, since the surface of a fixing member and a toner image are in contact under pressure in a heat-melted condition, there is a fear of an offset or winding phenomenon in which a part of the toner image is adhered to the surface of the above-described fixing member. In particular, in color toner fixation, since a toner should be fluidized more significantly by application of sufficient heat and pressure as compared with a monochrome toner fixation due to the necessity of melt-mixing of a plurality of toners having different colors, and further, since it is necessary to release thick toner layers composed of a plurality of toners having different colors without offset or winding phenomenon, releasing in color toner fixation is more difficult than releasing in monochrome toner fixation.
As a simple method for preventing adhesion of a toner to the surface of a fixing member, there is conducted a method in which silicone oil and the like as a liquid for preventing offset is coated on the surface of the fixing member. However, in the case of application of the oil and the like, adhesion of the oil to the transfer material and an image after fixation is problematical, further, there are problems that the fixation apparatus needs a tank which stores the oil and the like, accordingly, a reduction in the size of the apparatus is difficult, and replenishment of oil is complicated and restricts cost reduction, and the like.
Conventionally, the amount coated of the above-described oil and the like onto a general transfer material in color fixation is as large as about 8.0.times.10.sup.-2 mg/cm.sup.2, while in a monochrome printer, oil is not used at all, or if used, the amount coated of oil is 8.0.times.10.sup.-4 mg/cm.sup.2 or less which is one-hundredth of the amount coated of oil for color fixation, and the above-described defect is not problematical practically. Therefore, also in color fixation, is it eagerly desired that fixation is possible with the same amount of oil coated as for monochrome fixation. For this reason, there have been suggested various methods for enhancing the releasing property of a toner irrespective of fixing apparatuses by improvement of resins, wax and the like used for a toner.
For example, Japanese Patent Application Laid-Open (JP-A) No. 56-158340 and the like disclose monochrome toners which manifest an excellent oil-less fixing property by the effects of wax and a resin containing lower molecular weight components and higher molecular weight components and having a wide-spread molecular weight distribution. These monochrome toner resins are so designed that they can withstand the releasing force applied to a toner layer existing on the surface of a fixing apparatus, owing to rubber elasticity generated by the entangling of higher molecular weight components diluted by lower molecular weight components, namely offset is prevented.
However, for applying this technology to fixation of a color image, there are several problems. Namely, (1) by using as a binding resin one having rubber elasticity due to the entangling of higher molecular weight components, the gloss of the fixed image is reduced, and the color developing ability of a color image decreases, (2) a resin itself is flexible and easily deformed since it contains lower molecular weight components in the molecule even if the binding resin is elastic, therefore, when a toner layer is composed of 3 to 4 layers and the total thickness increases as in a color image, the toner layer easily winds around a fixing apparatus in being released to be deformed, and releasing property decreases, (3) in the case of a color image wherein a toner layer is composed of a plurality of layers, wax bleeds out also between toner layers having different hues, therefore, releasing between the toner layers, namely, offset easily occurs, and an offset preventing effect is not so easily obtained as in the fixation of a monochrome image, and the like.
Also regarding a color toner, although various fixation means containing higher molecular components and fixation means containing wax have been suggested, it is difficult to overcome the above-described problems, and though slight improvement in releasing property is obtained, there has been obtained no improvement at the level where no practical problems exist using the same oil coating amount as that for monochrome toner fixation, up to now.
Japanese Patent No. 2595239 discloses, as a toner used in a fixing apparatus using a fixing heating member constituted of a fluorine resin, a toner characterized regarding the viscoelasticity thereof in that when G' (dynamic storage modulus) is 17000 Pa, G" (dynamic loss elasticity) is from 17000 to 30000 Pa. This indicates an intermediate viscoelastic property between a conventional color toner mainly composed of lower molecular weight resins showing approximately viscous behavior and a monochrome toner showing rubber elasticity due to higher molecular weight components in a sufficiently melted condition of the toner at about 100 to 1000 Pa.s in terms of viscosity. Herein, polyester resins having wide molecular weight distribution, and styrene-acrylic resins having a Mw/Mn of as narrow as 3 or lower and a Mn of about 15000 or more are disclosed, as resins meeting the above-described viscoelastic conditions.
Therefore, if winding of a transfer material onto a fixing apparatus due to adhesion of a toner can be prevented, hot offset resistance is obtained with relatively suppressing color development by this viscoelastic property. However, in a resin of which molecular weight distribution is enlarged by simply combining higher molecular weight components with lower molecular weight components, a sufficient releasing property can not be obtained as described above, therefore, a large amount of oil to be coated is required to prevent winding. Further, the styrene-acrylic resins easily wind on a fixing apparatus and sufficient releasing ability is not obtained since modulus of rubber elasticity is low due to the resin composition even if the molecular weight thereof is increased.
Regarding addition of wax, when wax is added to a toner, releasing property tends to increase, however, there is a problem that light transmittance, charging property, toner powder flowability and the like tend to deteriorate. Particularly, in a toner obtained by a pulverizing method in which wax is exposed on the surface, deterioration of charging property and toner powder flowability tend to be remarkable. JP-A Nos. 5-61239, 7-92736, 7-159178 and the like disclose polymerization toners mainly composed of styrene-acrylic resins in which exposure of wax on the surface is suppressed by using suspension polymerization methods. Since these can prevent a reduction of surface properties and conditions caused by wax, flowability and releasing property of a toner powder are improved. However, since a styrene-acrylic resin is a main component, there is a tendency that winding easily occurs as described above, and this tendency becomes remarkable in the case of a color image wherein toners of various colors are laminated and the total toner layer thickness increases, and stable releasing is difficult.
In general, in conventional color fixation, a short life of a fixing roll due to reduction in the releasing property is pointed out as a significant problem.
As described above, it is difficult using conventional technologies to accomplish the oil-less fixing property of a toner attained now in a monochrome copying machine, in a color copying machine in which a higher image quality is required, and currently a new toner is required which is excellent in releasing property and can provide a color image having high quality.